Method and apparatus for history-based motion vector prediction with parallel processing

ABSTRACT

A method includes acquiring a current picture from a coded video bitstream, where the current picture is segmented into a plurality of units, each unit is divided into a plurality of blocks, and the plurality of blocks in each unit being arranged as a grid. The method further includes decoding, a current block from the plurality of blocks using an entry from a history motion vector (HMVP) buffer. The method further includes updating the HMVP buffer with a motion vector of the decoded current block. The method further includes determining whether a condition is satisfied, the condition specifying that (i) the current block is a beginning of a row included in the grid of the one of the units, and (ii) the plurality of blocks are decoded in accordance with a parallel process. The method further includes, in response to determining that the condition is satisfied, resetting the HMVP buffer.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/699,372, “TECHNIQUES FOR MOTION VECTORDIFFERENCE CODING IN BI-DIRECTIONAL MOTION COMPENSATION” filed on Jul.17, 2018, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding using inter-picture prediction with motioncompensation has been known for decades. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce aforementioned bandwidth or storage space requirements,in some cases by two orders of magnitude or more. Both lossless andlossy compression, as well as a combination thereof can be employed.Lossless compression refers to techniques where an exact copy of theoriginal signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between theoriginal and reconstructed signal is small enough to make thereconstructed signal useful for the intended application. In the case ofvideo, lossy compression is widely employed. The amount of distortiontolerated depends on the application; for example, users of certainconsumer streaming applications may tolerate higher distortion thanusers of television contribution applications. The compression ratioachievable can reflect that: higher allowable/tolerable distortion canyield higher compression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived fromneighboring area's MVs. That results in the MV found for a given area tobe similar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

A history buffer of motion vector predictors may be used to performencoding or decoding. Generally, the maintenance of the history bufferis performed after each block, in encoding or decoding order, iscompleted. If this block is coded in inter mode with a set of MVinformation, the MV of this block is put into HMVP buffer for updatingthe buffer. When encoding or decoding the current block, the MVpredictors for the current block may come from previously codedspatial/neighboring blocks. Some of these blocks may still be in theHMVP buffer. When putting a newly decoded/encoded MV into the HMVPbuffer, some comparisons may be performed to make sure the new MV isdifferent from all previous ones in the HMVP buffer. If there is alreadyan MV with the same value in the buffer, the old MV will be removed fromthe buffer, and the new MV is put into the buffer as the last entry.These general maintenance procedures of the history buffer do notproperly reset the history buffer when necessary to remove informationfrom the history buffer that may not be relevant for a current blockbeing encoded or decoded. Furthermore, encoding or decoding of a blockusing a history buffer doesn't properly take into account parallelprocessing of the block.

SUMMARY

An exemplary embodiment of the present disclosure includes a method forvideo decoding. The method includes acquiring a current picture from acoded video bitstream, where the current picture is segmented into aplurality of units, each unit is divided into a plurality of blocks, andthe plurality of blocks in each unit being arranged as a grid. Themethod further includes decoding, for one of the units, a current blockfrom the plurality of blocks using an entry from a history motion vector(HMVP) buffer. The method further includes updating the HMVP buffer witha motion vector of the decoded current block. The method furtherincludes determining whether a condition is satisfied, the conditionspecifying that (i) the current block is a beginning of a row includedin the grid of the one of the units, and (ii) the plurality of blocksare decoded in accordance with a parallel process. The method furtherincludes, in response to determining that the condition is satisfied,resetting the HMVP buffer.

An exemplary embodiment of the present disclosure includes a videodecoder. The video decoder includes processing circuitry configured toacquire a current picture from a coded video bitstream, where thecurrent picture is segmented into a plurality of units, each unit isdivided into a plurality of blocks, and the plurality of blocks in eachunit being arranged as a grid. The processing circuitry furtherconfigured to decode, for one of the units, a current block from theplurality of blocks using an entry from a history motion vector (HMVP)buffer. The processing circuitry further configured to update the HMVPbuffer with a motion vector of the decoded current block. The processingcircuitry further configured to determine whether a condition issatisfied, the condition specifying that (i) the current block is abeginning of a row included in the grid of the one of the units, and(ii) the plurality of blocks are decoded in accordance with a parallelprocess. The processing circuitry further configured to in response tothe determination that the condition is satisfied, reset the HMVPbuffer.

An exemplary embodiment of the present disclosure includesnon-transitory computer readable medium having instructions storedtherein, which when executed by a processor in a video decoder causesthe processor to execute a method. The method includes acquiring acurrent picture from a coded video bitstream, where the current pictureis segmented into a plurality of units, each unit is divided into aplurality of blocks, and the plurality of blocks in each unit beingarranged as a grid. The method further includes decoding, for one of theunits, a current block from the plurality of blocks using an entry froma history motion vector (HMVP) buffer. The method further includesupdating the HMVP buffer with a motion vector of the decoded currentblock. The method further includes determining whether a condition issatisfied, the condition specifying that (i) the current block is abeginning of a row included in the grid of the one of the units, and(ii) the plurality of blocks are decoded in accordance with a parallelprocess. The method further includes, in response to determining thatthe condition is satisfied, resetting the HMVP buffer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system (100) in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 6 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 7 is a schematic illustration of a current block and surroundingspatial merge candidates.

FIG. 8 illustrates example syntax of coding a motion vector difference(MVD).

FIG. 9 illustrates an example of predictive coding of a second MVD usinga first MVD as a predictor.

FIGS. 10A and 10B illustrate an embodiment of a history based motionvector prediction buffer.

FIG. 11 illustrates an example picture partitioned into coding treeunits.

FIG. 12 illustrates an example of a picture segmented into tiles.

FIG. 13 illustrates an embodiment of a process performed by an encoderor a decoder.

FIG. 14 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. Thecommunication system (100) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (150). Forexample, the communication system (100) includes a first pair ofterminal devices (110) and (120) interconnected via the network (150).In the FIG. 1 example, the first pair of terminal devices (110) and(120) performs unidirectional transmission of data. For example, theterminal device (110) may code video data (e.g., a stream of videopictures that are captured by the terminal device (110)) fortransmission to the other terminal device (120) via the network (150).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (120) may receive the codedvideo data from the network (150), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (100) includes a secondpair of terminal devices (130) and (140) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (130) and (140)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (130) and (140) via the network (150). Eachterminal device of the terminal devices (130) and (140) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (130) and (140), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 1 example, the terminal devices (110), (120), (130) and(140) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (150) represents any number ofnetworks that convey coded video data among the terminal devices (110),(120), (130) and (140), including for example wireline (wired) and/orwireless communication networks. The communication network (150) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(150) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating forexample a stream of video pictures (202) that are uncompressed. In anexample, the stream of video pictures (202) includes samples that aretaken by the digital camera. The stream of video pictures (202),depicted as a bold line to emphasize a high data volume when compared toencoded video data (204) (or coded video bitstreams), can be processedby an electronic device (220) that includes a video encoder (203)coupled to the video source (201). The video encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (204) (or encoded video bitstream (204)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (202), can be stored on a streamingserver (205) for future use. One or more streaming client subsystems,such as client subsystems (206) and (208) in FIG. 2 can access thestreaming server (205) to retrieve copies (207) and (209) of the encodedvideo data (204). A client subsystem (206) can include a video decoder(210), for example, in an electronic device (230). The video decoder(210) decodes the incoming copy (207) of the encoded video data andcreates an outgoing stream of video pictures (211) that can be renderedon a display (212) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (204),(207), and (209) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Codingor VVC. The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (220) and (230) can includeother components (not shown). For example, the electronic device (220)can include a video decoder (not shown) and the electronic device (230)can include a video encoder (not shown) as well.

FIG. 3 shows a block diagram of a video decoder (310) according to anembodiment of the present disclosure. The video decoder (310) can beincluded in an electronic device (330). The electronic device (330) caninclude a receiver (331) (e.g., receiving circuitry). The video decoder(310) can be used in the place of the video decoder (210) in the FIG. 2example.

The receiver (331) may receive one or more coded video sequences to bedecoded by the video decoder (310); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (301), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (331) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (331) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween the receiver (331) and an entropy decoder/parser (320) (“parser(320)” henceforth). In certain applications, the buffer memory (315) ispart of the video decoder (310). In others, it can be outside of thevideo decoder (310) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (310), forexample to combat network jitter, and in addition another buffer memory(315) inside the video decoder (310), for example to handle playouttiming. When the receiver (331) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (315) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (315) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (310).

The video decoder (310) may include the parser (320) to reconstructsymbols (321) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (310),and potentially information to control a rendering device such as arender device (312) (e.g., a display screen) that is not an integralpart of the electronic device (330) but can be coupled to the electronicdevice (330), as was shown in FIG. 3. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (320) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (320) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (320) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer memory (315), so as to createsymbols (321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (310)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). The scaler/inversetransform unit (351) can output blocks comprising sample values, thatcan be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (358). The currentpicture buffer (358) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(355), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (352) has generated to the outputsample information as provided by the scaler/inverse transform unit(351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (351) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (357) from where themotion compensation prediction unit (353) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (353) in the form of symbols (321) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (357) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (356) as symbols (321) from the parser (320), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (357) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (320)), the current picture buffer (358) can becomea part of the reference picture memory (357), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as document in thevideo compression technology or standard. Specifically, a profile canselect a certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (331) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 4 shows a block diagram of a video encoder (403) according to anembodiment of the present disclosure. The video encoder (403) isincluded in an electronic device (420). The electronic device (420)includes a transmitter (440) (e.g., transmitting circuitry). The videoencoder (403) can be used in the place of the video encoder (203) in theFIG. 2 example.

The video encoder (403) may receive video samples from a video source(401)(that is not part of the electronic device (420) in the FIG. 4example) that may capture video image(s) to be coded by the videoencoder (403). In another example, the video source (401) is a part ofthe electronic device (420).

The video source (401) may provide the source video sequence to be codedby the video encoder (403) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . )and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (401) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (401) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focusses on samples.

According to an embodiment, the video encoder (403) may code andcompress the pictures of the source video sequence into a coded videosequence (443) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (450). In some embodiments, the controller(450) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (450) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (450) can be configured to have other suitablefunctions that pertain to the video encoder (403) optimized for acertain system design.

In some embodiments, the video encoder (403) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (430) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (433)embedded in the video encoder (403). The decoder (433) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (434). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (434) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder, such as the video decoder (310), which has alreadybeen described in detail above in conjunction with FIG. 3. Brieflyreferring also to FIG. 3, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (445) and the parser (320) can be lossless, the entropy decodingparts of the video decoder (310), including the buffer memory (315), andparser (320) may not be fully implemented in the local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (430) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (432) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (433) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (434). In this manner, the video encoder(403) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new picture to be coded, the predictor(435) may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the source coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder (445)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (403) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the video encoder (403).During coding, the controller (450) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of Intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (403) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (403) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The source coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to Intra prediction) makes uses of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first and a second reference picture thatare both prior in decoding order to the current picture in the video(but may be in the past and future, respectively, in display order) areused. A block in the current picture can be coded by a first motionvector that points to a first reference block in the first referencepicture, and a second motion vector that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one coding unit (CU) of 64×64 pixels, or 4 CUs of 32×32pixels, or 16 CUs of 16×16 pixels. In an example, each CU is analyzed todetermine a prediction type for the CU, such as an inter prediction typeor an intra prediction type. The CU is split into one or more predictionunits (PUs) depending on the temporal and/or spatial predictability.Generally, each PU includes a luma prediction block (PB), and two chromaPBs. In an embodiment, a prediction operation in coding(encoding/decoding) is performed in the unit of a prediction block.Using a luma prediction block as an example of a prediction block, theprediction block includes a matrix of values (e.g., luma values) forpixels, such as 8×8 pixels, 16×16 pixels, 8×16 pixels, 16×8 pixels andthe like.

FIG. 5 shows a diagram of a video encoder (503) according to anotherembodiment of the disclosure. The video encoder (503) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (503) is used in theplace of the video encoder (203) in the FIG. 2 example.

In an HEVC example, the video encoder (503) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (503) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (503) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(503) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (503) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 5 example, the video encoder (503) includes the interencoder (530), an intra encoder (522), a residue calculator (523), aswitch (526), a residue encoder (524), a general controller (521) and anentropy encoder (525) coupled together as shown in FIG. 5.

The inter encoder (530) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique.

The intra encoder (522) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform and, in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques).

The general controller (521) is configured to determine general controldata and control other components of the video encoder (503) based onthe general control data. In an example, the general controller (521)determines the mode of the block, and provides a control signal to theswitch (526) based on the mode. For example, when the mode is the intra,the general controller (521) controls the switch (526) to select theintra mode result for use by the residue calculator (523), and controlsthe entropy encoder (525) to select the intra prediction information andinclude the intra prediction information in the bitstream; and when themode is the inter mode, the general controller (521) controls the switch(526) to select the inter prediction result for use by the residuecalculator (523), and controls the entropy encoder (525) to select theinter prediction information and include the inter predictioninformation in the bitstream.

The residue calculator (523) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (522) or the inter encoder (530). Theresidue encoder (524) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (524) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients.

The entropy encoder (525) is configured to format the bitstream toinclude the encoded block. The entropy encoder (525) is configured toinclude various information according to a suitable standard, such asHEVC standard. In an example, the entropy encoder (525) is configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. Note that, according to the disclosed subject matter, whencoding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 6 shows a diagram of a video decoder (610) according to anotherembodiment of the disclosure. The video decoder (610) is configured toreceive a coded pictures that are part of a coded video sequence, anddecode the coded picture to generate a reconstructed picture. In anexample, the video decoder (610) is used in the place of the videodecoder (210) in the FIG. 2 example.

In the FIG. 6 example, the video decoder (610) includes an entropydecoder (671), an inter decoder (680), a residue decoder (673), areconstruction module (674), and an intra decoder (672) coupled togetheras shown in FIG. 6.

The entropy decoder (671) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example,intra, inter, b-predicted, the latter two in merge submode or anothersubmode), prediction information (such as, for example, intra predictioninformation or inter prediction information) that can identify certainsample or metadata that is used for prediction by the intra decoder(672) or the inter decoder (680) respectively residual information inthe form of, for example, quantized transform coefficients, and thelike. In an example, when the prediction mode is inter or bi-predictedmode, the inter prediction information is provided to the inter decoder(680); and when the prediction type is the intra prediction type, theintra prediction information is provided to the intra decoder (672). Theresidual information can be subject to inverse quantization and isprovided to the residue decoder (673).

The inter decoder (680) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (672) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (673) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (673) mayalso require certain control information (to include the QuantizerParameter QP), and that information may be provided by the entropydecoder (671) (datapath not depicted as this may be low volume controlinformation only).

The reconstruction module (674) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (673) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (203), (403) and (503), and thevideo decoders (210), (310) and (610) can be implemented using anysuitable technique. In an embodiment, the video encoders (203), (403)and (503), and the video decoders (210), (310) and (610) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (203), (403) and (403), and the videodecoders (210), (310) and (610) can be implemented using one or moreprocessors that execute software instructions.

According to some embodiments, a motion vector (MV) for a block can becoded either in an explicit way, to signal the difference between amotion vector predictor, or in an implicit way, to be indicated asderived from one previously coded or generated motion vector, or motionvector pair if coded using bi-directional prediction. The implicitcoding of a motion vector may be referred to as merge mode, where acurrent block is merged into a previously coded block by sharing themotion information of the previously coded block.

Merge candidates may be formed by checking motion information fromeither spatial or temporal neighbouring blocks of the current block.Referring to FIG. 7, a current block (701) comprises samples that havebeen found by the encoder/decoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. In some embodiments, instead of coding that motionvector directly, the motion vector can be derived from metadataassociated with one or more reference pictures, for example, from a mostrecent (in decoding order) reference picture, using the motion vectorassociated with either one of five surrounding samples, denoted D, A, C,B, and E (702 through 706, respectively). The blocks A, B, C, D, and Emay be referred to as spatial merge candidates. These candidates may besequentially checked into a merge candidate list. A pruning operationmay be performed to make sure duplicated candidates are removed from thelist.

In some embodiments, after putting spatial candidates into the mergelist, temporal candidates are also checked into the list. For example, acurrent block's collocated block in a specified reference picture isfound. The motion information at the C0 position (707) in the referencepicture is used as a temporal merge candidate. The C0 position may be ablock in the reference picture in which the top left corner of thisblock is at a bottom right corner of a collocated block in the referencepicture of the current block 701. The collocated block in the referencepicture may include the same position coordinates (e.g., x and ycoordinates) as the current block 701. If the block at the C0 position(707) is not coded in an inter mode or is not available, the block atthe C1 position may be used. The block at the C1 position may have a topleft corner at a center location (e.g., w/2, h/2) of a block within thecollocated block in the reference picture. Particularly, the block atposition C1 may be a sub-block of the collocated block in the referencepicture. In the above example, w and h are the width and height of theblock, respectively. According to some embodiments, additional mergecandidates include combined bi-predictive candidates and zero motionvector candidates.

According to some embodiments, Advanced Motion Vector Prediction (AMVP)uses spatial and temporal neighboring blocks' motion information topredict the motion information of a current block, while the predictionresidue is further coded. The AMVP mode may also be referred to as theresidue mode. FIG. 7 illustrates examples of spatial and temporalneighboring candidates. In an example of the AMVP mode, a two-candidatemotion vector predictor list is formed. The first candidate predictor isfrom a first available motion vector from the left edge in the order ofthe spatial A0 and A1 positions. The second candidate predictor is froma first available motion vector from the top edge, in the order ofspatial B0, B1, and B2 positions. If no valid motion vector is foundfrom the checked locations for either the left edge or the top edge, nocandidate will be filled in the motion vector predictor list. If the twocandidates are available and are the same, one candidate is kept in themotion vector predictor list. If the motion vector predictor list is notfull with two different candidates, the temporal collocated block'smotion vector in the reference picture, after scaling, at the C0location is used as another candidate. If motion information at thecollocated block of the C0 location in the reference picture is notavailable, the collocated block of location C1 in the reference pictureis used instead. If, after checking the spatial and temporal candidates,there are still not enough motion vector predictor candidates, the zeromotion vector is used to fill up the motion vector predictor list.

According to some embodiments, in AMVP mode, after a motion vector ispredicted by a MV prediction, the residue part is referred as a motionvector difference (MVD), which also has x and y components. The codingof the MVD may involve (i) a binarization of a difference value in eachcomponent and (ii) context modeling for some of the binarized bins.

In a bi-directional predicted slice (B_slice), each block may be codedin either a forward prediction (i.e., predicted from a reference picturein list 0 or L0), a backward prediction (i.e., predicted from areference picture in list 1 or L1), or a bi-directional prediction(i.e., predicted from two reference pictures one in list 0 and one inlist 1). The former two cases may also be referred to as auni-directional prediction. In some embodiments, if the bi-directionalprediction is used in a coded block, there is a pair of motion vectorspointing to the two reference pictures. Furthermore, there is a pair ofMVDs to be coded, which may be coded independently by existing videocoding standards. FIG. 8 illustrates example syntax in which the pair ofMVDs are coded independently.

Embodiments of the present disclosure include improved techniques forMVD coding by exploring correlations between the pair of MVDs whenbi-directional prediction is used. The embodiments of the presentdisclosure improve coding efficiency in the bi-directional prediction.In this regard, the MVD of a first MV is used to predict the MVD of asecond MV. After being predicted by the first MVD, the second MVDresidue is further coded using an original MVD coding module, or amodified MVD coding module.

According to some embodiments, more than one motion vector in a blockneeds to be coded, the MVD of a first vector is used as a predictor topredict the MVD(s) of the other MVs in the block. Using thebi-directional mode as an example, where a pair of MVs are to be coded,the MVD1 is predicted by MVD0, before performing entropy coding for thisMV difference.

FIG. 9 illustrates an embodiment of a prediction scheme, where MVx isthe motion vector in List x (x=0,1), MVPx is the motion vector predictorin List x, and MVDx is the motion vector difference in List x. Asillustrated in FIG. 9, the second MVD (i.e., MVD1) is predicted usingthe first MVD (i.e., MVD0) as a predictor. Each of MVP0 and MVP1 maycome from spatial or temporal neighboring blocks' motion vectors. Whenthere are more than 1 motion vector candidates, for each of MVP0 andMVP1, an index may be signaled to choose from the candidate list.

According to some embodiments, conditions on whether the MVD1 ispredicted from MVD0 may be specified. For example, MVD1 may be predictedfrom MVD0 only when (i) the reference picture from L0 has a pictureorder count (POC) number smaller than a POC of a current picture, and(ii) the reference picture from L1 has a POC number larger than the POCof the current picture. In another example, MVD1 may be predicted fromMVD0 only when (ii) the reference picture from L0 has a POC numberlarger than the POC of the current picture, and (ii) the referencepicture from L1 has a POC number smaller than the POC of the currentpicture.

In some embodiments, both MVD0 and MVD1 go through a same MVD entropycoding module that is designed also for other situations where only oneMVD coding is performed. In another embodiment, the MVD0 goes through asame MVD entropy coding module that is designed also for othersituations where only one MVD coding is performed.

Embodiments of the present disclosure disclose several methods ofgetting motion vector predictors for inter-picture prediction coding.These methods include storing N previously coded blocks' MV predictorsin a history-based MV (HMVP) buffer. This buffer with multiple HMVPcandidates is maintained during the encoding/decoding process. Thebuffer may operate in a first-in-first-out (FIFO) principle such thatthe most recent coded motion information is firstly considered when thisbuffer is used during motion vector prediction.

These methods may be applied to both merge mode or AMVP mode. Theembodiments of the present disclosure may be extended to any videocoding method that uses the merge and general MV prediction concepts.Embodiments of the present disclosure may also be applied to the skipmode since this mode uses the merge mode to derive the motioninformation.

FIGS. 10A and 10B illustrate an HMVP buffer before and after a candidateis inserted, respectively. As illustrated in FIGS. 10A and 10B, the HMVPbuffer includes 5 entries with the index [0] to [4]. In FIG. 10B, theentry CL_0 is inserted at index [4], which causes the other entries tomove to the left by one, resulting in the entry HMPV_0 being removedfrom the buffer. The entry CL_0 may include motion vector predictorinformation of a previously encoded or decoded block.

According to some embodiments, the HMVP buffer is emptied or reset to azero state when a condition is satisfied. The condition may be that (i)the current CU is the beginning of a CTU row and (ii) the current blockis encoded/decoded using wavefront parallel processing. In wavefrontparallel processing, before the encoding or decoding of a current row iscompleted, the encoding or decoding of another row may be started.

According to some embodiments, an HMVP_row buffer with the same size ofthe HMVP buffer, is used to store the entries of the HMVP buffer, afterthe first CTU of every CTU row is completed. Accordingly, at thebeginning of a new CTU row, the HMVP buffer may be filled with theinformation in the HMVP_row buffer. By resetting the HMVP buffer at theend of the CTU row, and copying the contents of the HMVP_row buffer tothe HMVP buffer, the blocks of the first CTU being decoded may bedecoded with information from the CTU directly above the first CTU.

In some embodiments, for each tile in a picture, the HMVP_row buffer isused to store the HMVP information after the first CTU of each tile rowis finished. Accordingly, for the first CTU of a new tile row, the HMVPbuffer may be filled using the information from the HMVP_row buffer. Insome embodiments, the HMVP_row buffer is initiated to a zero state atthe beginning of a first CTU row of a tile or slice.

FIG. 11 illustrates an example picture 1100 that is divided into CTUsCTU_00 to CTU_23. CTUs CTU_00 to CTU_03 are in the first CTU rowCTU_Row_[0]. CTUs CTU_10 to CTU_13 are in the second CTU rowCTU_Row_[1]. CTUs CTU_20 to CTU_23 are in the third CTU_row. Each CTU inthe picture 1100 may be further divided into a plurality of blocks. Theblocks may be CUs or coding blocks (CBs).

In some embodiments, when wavefront parallel processing is used toencode or decode the picture 1100, at least two processor threads may beused. Each processor thread may have an associated HMVP buffer, and ashared row buffer. For example, referring to picture 1100, processorthreads PT_1 and PT_2 may be used for the encoding or decoding ofpicture 1100. Processor thread PT_1 may be associated with historybuffer HMVP_1, and processor thread PT_2 may be associated with historybuffer HMVP_2. Furthermore, each of processor threads PT_1 and PT_2 mayshare the same history row buffer.

In some embodiments, before the first block in the first CTU (e.g.,CTU_00) of picture 1100 is encoded or decoded, the associated HMVPbuffer of the processor thread that encodes or decodes the CTUs in thefirst CTU row is loaded with initial values. The initial values may bestored in the memory of an encoder or decoder. In an another example,the associated HMVP buffer of the processor thread that encodes ordecodes the CTUs in the first CTU row may initialized to a zero state.Additionally, the HMVP_row buffer may be initialized to a zero statebefore the first block in the first CTU (e.g., CTU_00) of picture 1100is encoded or decoded.

Processor thread PT_1 may be used to start encoding the CTUs inCTU_Row_[0]. When the last block of CTU_00 (e.g., first CTU inCTU_Row_[0]) is encoded or decoded by PT_1, the contents of the bufferHMVP_1 are copied to the HMVP_row buffer. When the last block of CTU_03is encoded or decoded (e.g., last CTU in CTU_Row_[0]) by PT_1 the bufferHMVP_1 is emptied.

Furthermore, when the last block of CTU_01 is encoded or decoded byPT_1, the contents of the HMVP_row buffer may be copied to the bufferHMVP_2, and the encoding or decoding of the CTUs in the next CTU row(i.e., CTU_Row_[1]) may by started by the second processor thread PT_2.In this regard, processor thread PT_2 starts encoding or decoding theCTUs of CTU_Row_[1] while the processor thread PT_1 is still encoding ordecoding the CTUs of CTU_Row_[0]. When the last block of CTU_10 (e.g.,first CTU in CTU_Row_[1]) is encoded or decoded by PT_2, the contents ofthe buffer HMVP_2 are copied to the HMVP_row buffer. When the last blockof CTU_13 is encoded or decoded (e.g., last CTU in CTU_Row_[1]) by PT_2,the buffer HMVP_2 is emptied.

Additionally, when the first processor thread PT_1 has finished encodingor decoding the CTUs CTU_Row_[0], and after the contents of bufferHMVP_2 are copied to the HMVP_row buffer, the contents of the HMVP_rowbuffer are copied to the buffer HMVP_1, and the encoding or decoding ofthe CTUs in CTU_row[2] may be started by the first processor threadPT_1. In this regard, processor thread PT_1 starts encoding or decodingthe CTUs of CTU_Row_[2] while the processor thread PT_2 is stillencoding or decoding the CTUs of CTU_Row_[1].

Accordingly, in the above example, by resetting the contents of buffersHMVP_1 and HMVP_2 and copying the contents of the HMVP_row buffer tobuffers HMVP_1 and HMVP_2 as discussed above, the significantlyadvantageous features of (i) the encoding or decoding of the blocks inthe first CTU of each row with relevant motion information, and (ii) theencoding and decoding of CTU rows in parallel is achieved.

FIG. 12 illustrates an example of picture 1100 divided into two tilesTile_1 and Tile_2. When wavefront parallel processing is used whenencoding or decoding picture 1100, Tile_1 and Tile_2 may have separateprocessor threads. For example, Tile_1 may have processor threadsTile_1_PT_1 and Tile_1_PT_2, where these processor threads areassociated with buffers Tile_1_HMVP_1 and Tile_1_HMVP_2, respectively.Tile_2 may have processor threads Tile_2_PT_1 and Tile_2_PT_2, wherethese processor threads are associated with buffers Tile_2_HMVP_1 andTile_2_HMVP_2, respectively. Furthermore, the processor threads of Tile1 may use a shared row buffer HMVP_row_1 buffer, and the processorthreads of Tile 2 may use the shared row buffer HMVP_row_2.

As an example, processor thread Tile_1_PT_1 is used to process the CTUsin Tile_Row_[0] of Tile 1. In some embodiments, before the first blockin the first CTU of Tile_1 (e.g., CTU_00) is encoded or decoded, thebuffer Tile_1_HMVP_1 is loaded with initial values. The initial valuesmay be stored in the memory of an encoder or decoder. In anotherexample, the buffer Tile_1_HMVP_1 may be initialized to a zero state.Additionally, the row buffer HMVP_row_1 may be initialized to a zerostate before the first block in the first CTU (e.g., CTU_00) of Tile isencoded or decoded. When the last block of CTU_00 (e.g., first CTU inCTU_Row_[0] of Tile 1) is encoded or decoded by Tile_1_PT_1, thecontents of the buffer Tile_1_HMVP_1 is copied to the row bufferHMVP_row_1. When the last block of CTU_01 is encoded or decoded (e.g.,last CTU in CTU_Row_[0] of Tile 1), the buffer Tile_1_HMVP_1 is reset.Furthermore, when the last block of CTU_00 is encoded or decoded, thecontents of the row buffer HMVP_row_1 are copied to the bufferTile_1_HMVP_2, and the processor thread Tile_1_PT_2 may start theencoding or decoding of the CTUs in Tile_Row_[1] of Tile 1 while theprocessor thread Tile_1_PT_1 is still encoding or decoding the CTUs oftile row Tile Row[0] of Tile 1.

Tile_2 may be encoded or decoded in parallel with Tile_1. As an example,processor thread Tile_2_PT_1 is used to process the CTUs in Tile_Row_[0]of Tile 2. The buffers Tile_2_HMVP_1 and HMVP_row_2 may be initializedin the same manner as the buffers Tile_1_HMVP_1 and HMVO_row_1,respectively, as described above. When the last block of CTU_02 (e.g.,first CTU in CTU_Row_[0] of Tile 2) is encoded or decoded byTile_2_PT_1, the contents of the buffer Tile_2_HMVP_1 is copied to therow buffer HMVP_row_2. When the last block of CTU_03 is encoded ordecoded (e.g., last CTU in CTU_Row_[0] of Tile 2) by Tile_2_PT_1, thebuffer Tile_2_HMVP_1 is reset. Furthermore, when the last block ofCTU_02 is encoded or decoded, the contents of the row buffer HMVP_row_2are copied to the buffer Tile_2_HMVP_2, and the processor threadTile_2_PT_2 may start the encoding or decoding of the CTUs inTile_Row_[1] of Tile 2 while the processor thread Tile_1_PT_1 is stillencoding or decoding the CTUs of tile row Tile Row[0] of Tile 2.

FIG. 13 illustrates an embodiment of a process performed by an encodersuch as encoder 503 or a decoder such as decoder 610. The process maystart at step S1300 where a current picture is acquired from a videobitstream. For example, picture 1100 (FIG. 11) may be the acquiredpicture. The process proceeds to step S1302 where a current block fromthe current picture is encoded/decoded using one or more entries from anHMVP buffer. For example, referring to picture 1100, if the first blockin CTU_00 is being encoded/decoded, the buffer HMVP_1 may be initializedto an initial state, and the first block may be encoded/decoded with oneor more entries from the buffer HMVP_1 after this buffer is initialized.The process proceeds to step S1304 where the HMVP buffer is updated withmotion vector information of the encoded/decoded current block.

The process proceeds to step S1306 where it is determined whetherparallel processing is enabled for the current picture. For example, itmay be determined that wavefront parallel processing is enabled for thepicture 1100. If parallel processing is not enabled for the currentpicture, the process proceeds to step S1316, which is described infurther detail below.

If parallel processing is enabled for the current picture, the processproceeds to step S1308 to determine whether the current encoded/decodedblock is at the end of the CTU row. For example, referring to picture1100, if the current block that is encoded/decoded is the last block ofCTU_03, then the current encoded/decoded block is at the end ofCTU_Row_[0]. If the current encoded/decoded block is at the end of theCTU row, the process proceeds to step S1310 where the HMVP buffer isreset (e.g., emptied). For example, if processor thread PT_1 is used toencode or decode the CTUs of CTU_Row_[0], the buffer HMVP_1 is resetafter the last block of CTU_03 is processed. The process proceeds fromstep S1310 to step S1316, which is described in further detail below.

If the current encoded/decoded block is not the end of the CTU row, theprocess proceeds to step S1312 to determine whether the currentencoded/decoded block is the last block of the first CTU in the CTU row.If the current encoded/decoded block is the last block of the first CTUin the CTU row, the process proceeds to step S1314 where the contents ofthe HMVP buffer are copied into the HVMP_row buffer. For example,referring to picture 1100, if the current encoded/decoded block is thelast block of CTU_00, the contents of the buffer HVMP_1 buffer arecopied into the contents of the HMVP_row buffer before the first blockof CTU_01 is encoded/decoded. As discussed above, after the last blockof CTU_01 is encoded/decoded, the contents of the HMVP_row buffer arecopied to the buffer HMVP_2, where the processor thread PT_2 can startencoding/decoding the CTUs of CTU_Row_[1] while the processor thread isstill encoding or decoding the CTUs of CTU_Row_[0]. The process proceedsfrom step S1314 to step S1316, which is described in further detailbelow.

If the current encoded/decoded block is not the last block of the firstCTU in the CTU row, the process proceeds to step S1316 to determine ifthe current encoded/decoded block is the last block in the acquiredpicture. If the current encoded/decoded block is the last block in theacquired picture, the process in FIG. 13 ends. For example, if thecurrent encoded/decoded block is the last block of CTU_23, the processin FIG. 13 is completed. If the current encoded/decoded block is not thelast block in the acquired picture, the process returns from step S1316to step S1302.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 14 shows a computersystem (1400) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 14 for computer system (1400) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1400).

Computer system (1400) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1401), mouse (1402), trackpad (1403), touchscreen (1410), data-glove (not shown), joystick (1405), microphone(1406), scanner (1407), camera (1408).

Computer system (1400) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1410), data-glove (not shown), or joystick (1405), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1409), headphones(not depicted)), visual output devices (such as screens (1410) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1400) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1420) with CD/DVD or the like media (1421), thumb-drive (1422),removable hard drive or solid state drive (1423), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1400) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1449) (such as, for example USB ports of thecomputer system (1400)); others are commonly integrated into the core ofthe computer system (1400) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1400) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1440) of thecomputer system (1400).

The core (1440) can include one or more Central Processing Units (CPU)(1441), Graphics Processing Units (GPU) (1442), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1443), hardware accelerators for certain tasks (1444), and so forth.These devices, along with Read-only memory (ROM) (1445), Random-accessmemory (1446), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1447), may be connectedthrough a system bus (1448). In some computer systems, the system bus(1448) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1448),or through a peripheral bus (1449). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1441), GPUs (1442), FPGAs (1443), and accelerators (1444) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1445) or RAM (1446). Transitional data can be also be stored in RAM(1446), whereas permanent data can be stored for example, in theinternal mass storage (1447). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1441), GPU (1442), massstorage (1447), ROM (1445), RAM (1446), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1400), and specifically the core (1440) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1440) that are of non-transitorynature, such as core-internal mass storage (1447) or ROM (1445). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1440). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1440) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1446) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1444)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

MV: Motion Vector

HEVC: High Efficiency Video Coding

SEI: Supplementary Enhancement Information

VUI: Video Usability Information

GOPs: Groups of Pictures

TUs: Transform Units,

PUs: Prediction Units

CTUs: Coding Tree Units

CTBs: Coding Tree Blocks

PBs: Prediction Blocks

HRD: Hypothetical Reference Decoder

SNR: Signal Noise Ratio

CPUs: Central Processing Units

GPUs: Graphics Processing Units

CRT: Cathode Ray Tube

LCD: Liquid-Crystal Display

OLED: Organic Light-Emitting Diode

CD: Compact Disc

DVD: Digital Video Disc

ROM: Read-Only Memory

RAM: Random Access Memory

ASIC: Application-Specific Integrated Circuit

PLD: Programmable Logic Device

LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution

CANBus: Controller Area Network Bus

USB: Universal Serial Bus

PCI: Peripheral Component Interconnect

FPGA: Field Programmable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

(1) A method of video decoding for a decoder, the method includingacquiring a current picture from a coded video bitstream, the currentpicture being segmented into a plurality of units, each unit dividedinto a plurality of blocks, the plurality of blocks in each unit beingarranged as a grid; decoding, for one of the units, a current block fromthe plurality of blocks using an entry from a history motion vector(HMVP) buffer; updating the HMVP buffer with a motion vector of thedecoded current block; determining whether a condition is satisfied, thecondition specifying that (i) the current block is a beginning of a rowincluded in the grid of the one of the units, and (ii) the plurality ofblocks are decoded in accordance with a parallel process; and inresponse to determining that the condition is satisfied, resetting theHMVP buffer.

(2) The method according to feature (1), further including determiningwhether the current block is a last block of a first unit of the row;and in response to determining that the current block is the last blockof the first unit of the row, copying contents of the HMVP buffer into arow buffer.

(3) The method according to feature (2), further including in responseto determining that the current block is a last block of a second unitof the row, copying contents of the row buffer into another HMVP bufferfor parallel decoding of the next row.

(4) The method according to any one of features (1)-(3), in which theHMVP buffer is a first-in-first-out (FIFO) buffer, and in which theupdating the HMVP buffer with the motion vector includes storing themotion vector at a last entry of the HMVP buffer and deleting a firstentry of the HMVP buffer.

(5) The method according to any one of features (1)-(4), in which theunit is a coding tree unit (CTU).

(6) The method according to any one of features (2)-(5), in which theunit is a tile, the decoded one of the units is a first tile, and thefirst tile and a second tile from the plurality of units are decoded inparallel.

(7) A video decoder for video decoding including processing circuitryconfigured to: acquire a current picture from a coded video bitstream,the current picture being segmented into a plurality of units, each unitdivided into a plurality of blocks, the plurality of blocks in each unitbeing arranged as a grid, decode, for one of the units, a current blockfrom the plurality of blocks using an entry from a history motion vector(HMVP) buffer, update the HMVP buffer with a motion vector of thedecoded current block, determine whether a condition is satisfied, thecondition specifying that (i) the current block is a beginning of a rowincluded in the grid of the one of the units, and (ii) the plurality ofblocks are decoded in accordance with a parallel process, and inresponse to the determination that the condition is satisfied, reset theHMVP buffer.

(8) The video decoder according to feature (7), in which the processingcircuitry is further configured to: determine whether the current blockis a last block of a first unit of the row, and in response to thedetermination that the current block is the last block of the first unitof the row, copying contents of the HMVP buffer into a row buffer.

(9) The video decoder according to feature (8), in which the processingcircuitry is further configured to: in response to the determinationthat the current block is a last block of a second unit of the row,copying contents of the row buffer into another HMVP buffer for paralleldecoding of the next row.

(10) The video decoder according to any one of features (7)-(9), inwhich the HMVP buffer is a first-in-first-out (FIFO) buffer, and inwhich the updating the HMVP buffer with the motion vector includesstoring the motion vector at a last entry of the HMVP buffer anddeleting a first entry of the HMVP buffer.

(11) The video decoder according to any one of features (7)-(10), inwhich the unit is a coding tree unit (CTU).

(12) The video decoder according to any one of features (8)-(11), inwhich the unit is a tile, the decoded one of the units is a first tile,and the first tile and a second tile from the plurality of units aredecoded in parallel.

(13) A non-transitory computer readable medium having instructionsstored therein, which when executed by a processor in a video decodercauses the processor to execute a method including acquiring a currentpicture from a coded video bitstream, the current picture beingsegmented into a plurality of units, each unit divided into a pluralityof blocks, the plurality of blocks in each unit being arranged as agrid; decoding, for one of the units, a current block from the pluralityof blocks using an entry from a history motion vector (HMVP) buffer;updating the HMVP buffer with a motion vector of the decoded currentblock; determining whether a condition is satisfied, the conditionspecifying that (i) the current block is a beginning of a row includedin the grid of the one of the units, and (ii) the plurality of blocksare decoded in accordance with a parallel process; and in response todetermining that the condition is satisfied, resetting the HMVP buffer.

(14) The non-transitory computer readable medium according to feature(13), the method further including determining whether the current blockis a last block of a first unit of the row; and in response todetermining that the current block is the last block of the first unitof the row, copying contents of the HMVP buffer into a row buffer.

(15) The non-transitory computer readable medium according to feature(14), the method further comprising: in response to determining that thecurrent block is a last block of a second unit of the row, copyingcontents of the row buffer into another HMVP buffer for paralleldecoding of the next row.

(16) The non-transitory computer readable medium according to any one offeatures (13)-(15), in which the HMVP buffer is a first-in-first-out(FIFO) buffer, and in which the updating the HMVP buffer with the motionvector includes storing the motion vector at a last entry of the HMVPbuffer and deleting a first entry of the HMVP buffer.

(17) The non-transitory computer readable medium according to any one offeatures (13)-(16), in which the unit is a coding tree unit (CTU).

(18) The non-transitory computer readable medium according to any one offeatures (14)-(17), in which the unit is a tile, the decoded one of theunits is a first tile, and the first tile and a second tile from theplurality of units are decoded in parallel.

What is claimed is:
 1. A method of video decoding for a decoder, themethod comprising: acquiring a current picture from a coded videobitstream, the current picture being segmented into a plurality ofunits, each unit divided into a plurality of blocks, the plurality ofblocks in each unit being arranged as a grid in each unit, and theplurality of units being arranged as a grid in the current picture;decoding, for one of the units, a current block from the plurality ofblocks using an entry from a history motion vector (HMVP) buffer;updating the HMVP buffer with a motion vector of the decoded currentblock; determining whether a condition is satisfied, the conditionspecifying that (i) the current block is a first block in a first row ofa grid in a first unit of a row of units of the grid in the currentpicture, and (ii) the plurality of units blocks are decoded inaccordance with a parallel process; in response to determining that thecondition is satisfied, resetting the HMVP buffer; determining whetherthe current block is a last block in a last row of the grid in the firstunit of the row of units of the grid in the current picture; and inresponse to determining that the current block is the last block in thelast row of the grid in the first unit of the row of units of the gridin the current picture, copying contents of the HMVP buffer into a rowbuffer.
 2. The method according to claim 1, further comprising: inresponse to determining that the current block is a last block in a lastrow of a grid in a second unit of the row of units of the grid in thecurrent picture, copying contents of the row buffer into another HMVPbuffer for parallel decoding of the units in the next row.
 3. The methodaccording to claim 1, wherein the HMVP buffer is a first-in-first-out(FIFO) buffer, and wherein the updating the HMVP buffer with the motionvector includes storing the motion vector at a last entry of the HMVPbuffer and deleting a first entry of the HMVP buffer.
 4. The methodaccording to claim 1, wherein the unit is a coding tree unit (CTU). 5.The method according to claim 1, wherein the unit is a tile, the decodedone of the units is a first tile, and the first tile and a second tilefrom the plurality of units are decoded in parallel.
 6. A video decoderfor video decoding, comprising: processing circuitry configured to:acquire a current picture from a coded video bitstream, the currentpicture being segmented into a plurality of units, each unit dividedinto a plurality of blocks, the plurality of blocks in each unit beingarranged as a grid in each unit, and the plurality of units beingarranged as a grid in the current picture, decode, for one of the units,a current block from the plurality of blocks using an entry from ahistory motion vector (HMVP) buffer, update the HMVP buffer with amotion vector of the decoded current block, determine whether acondition is satisfied, the condition specifying that (i) the currentblock is a first block in a first row of a grid in a first unit of a rowof units of the grid in the current picture, and (ii) the plurality ofunits are decoded in accordance with a parallel process, in response tothe determination that the condition is satisfied, reset the HMVPbuffer, determine whether the current block is a last block in a lastrow of the grid in the first unit of the row of units of the grid in thecurrent picture, and in response to the determination that the currentblock is the last block in the last row of the grid in the first unit ofthe row of units of the grid in the current picture, copying contents ofthe HMVP buffer into a row buffer.
 7. The video decoder according toclaim 6, wherein the processing circuitry is further configured to: inresponse to the determination that the current block is a last block ina last row of a grid in a second unit of the row of units of the grid inthe current picture, copying contents of the row buffer into anotherHMVP buffer for parallel decoding of the units in the next row.
 8. Thevideo decoder according to claim 6, wherein the HMVP buffer is afirst-in-first-out (FIFO) buffer, and wherein the updating the HMVPbuffer with the motion vector includes storing the motion vector at alast entry of the HMVP buffer and deleting a first entry of the HMVPbuffer.
 9. The video decoder according to claim 6, wherein the unit is acoding tree unit (CTU).
 10. The video decoder according to claim 6,wherein the unit is a tile, the decoded one of the units is a firsttile, and the first tile and a second tile from the plurality of unitsare decoded in parallel.
 11. A non-transitory computer readable mediumhaving instructions stored therein, which when executed by a processorin a video decoder causes the processor to execute a method comprising:acquiring a current picture from a coded video bitstream, the currentpicture being segmented into a plurality of units, each unit dividedinto a plurality of blocks, the plurality of blocks in each unit beingarranged as a grid in each unit, and the plurality of units beingarranged as a grid in the current picture; decoding, for one of theunits, a current block from the plurality of blocks using an entry froma history motion vector (HMVP) buffer; updating the HMVP buffer with amotion vector of the decoded current block; determining whether acondition is satisfied, the condition specifying that (i) the currentblock is a first block in a first row of a grid in a first unit of a rowof units of the grid in the current picture, and (ii) the plurality ofunits blocks are decoded in accordance with a parallel process; inresponse to determining that the condition is satisfied, resetting theHMVP buffer; determining whether the current block is a last block in alast row of the grid in the first unit of the row of units of the gridin the current picture; and in response to determining that the currentblock is the last block in the last row of the grid in the first unit ofthe row of units of the grid in the current picture, copying contents ofthe HMVP buffer into a row buffer.
 12. The non-transitory computerreadable medium according to claim 11, the method further comprising: inresponse to determining that the current block is a last block in a lastrow of a grid in a second unit of the row of units of the grid in thecurrent picture, copying contents of the row buffer into another HMVPbuffer for parallel decoding of the units in the next row.
 13. Thenon-transitory computer readable medium according to claim 11, whereinthe HMVP buffer is a first-in-first-out (FIFO) buffer, and wherein theupdating the HMVP buffer with the motion vector includes storing themotion vector at a last entry of the HMVP buffer and deleting a firstentry of the HMVP buffer.
 14. The non-transitory computer readablemedium according to claim 11, wherein the unit is a coding tree unit(CTU).
 15. The non-transitory computer readable medium according toclaim 11, wherein the unit is a tile, the decoded one of the units is afirst tile, and the first tile and a second tile from the plurality ofunits are decoded in parallel.